Proliferation and differentiation of cells, and responses of cells to various stimuli are strictly regulated by various growth factors. These growth factors are known to act via receptors which are specific to the above growth factors (Nicola, N. A., Annu. Rev. Biochem. 58, 45, 1989; Lowenberg, B., Blood 81, 281, 1991). Of those receptors, the receptors containing a tyrosine kinase domain are classified as receptor tyrosine kinases (RTKs).
RTKs comprise an extracellular region, a transmembrane region, as well as an intracellular region containing a tyrosine kinase domain and a juxtamembrane between the transmembrane region and the tyrosine kinase domain, and further roughly classified into four types according to structural characteristics and amino acid sequence homology.
Type I receptors have a monomeric structure, with two cysteine-rich repeat sequences in their extracellular region, and are exemplified by the EGF receptor, HER2/neu and the like.
Type II receptors have a structure comprising two subunits each for α and β, which are bound via S—S bond, wherein the α chain is an extracellular region containing one cysteine-rich repeat sequence, and wherein the β chain has a transmembrane region, a juxtamembrane, and a tyrosine kinase domain. Examples are an insulin receptor and an IGF-1 receptor.
Type III receptors have a monomeric structure containing five immunoglobulin-like cysteine-rich sequences in their extracellular region and two tyrosine kinase domains interrupted by a kinase insert in their intracellular region. Examples are PDGF receptor, fms (CSF-1 receptor), kit (SLF receptor) and the like.
Type IV receptors resemble type III receptors but have three immunoglobulin-like repeat sequences, and are exemplified by FGF receptor.
fms-Like tyrosine kinase 3 (hereinafter abbreviated as FLT3; Matthews, W., Cell 65, 1143, 1991; Rosnet, O., Genomics 9, 380, 1991), which is expressed in leukemic cells etc., also referred to as fetal liver kinase 2 (FLK2) or STK-1, is known to as type III receptors (Small, D., Proc. Natl. Acad. Sci. USA 91, 459, 1993; Lyman, S. D., Oncogene 8, 815, 1993; Rosnet, O., Blood 82, 1110, 1993; Agnes, F., Gene 145, 283, 1994).
In these receptor tyrosine kinases, aggregation, such as dimerization, takes place upon binding of a ligand, such as a growth factor, to the extracellular region, thereby resulting in the activation of kinase. Although in these tyrosine kinases, their ligands have been first found and then their receptors in most cases, there are receptors of which ligands remain unknown.
Regarding FLT3, which has been remarked in proliferation mechanism of hematopoietic stem cells and leukemia, after finding the FLT3, the FLT3 ligand has been found (Lyman, S. D., Cell 75, 1157, 1993; Hannum, C., Nature 368, 643, 1994). Since the FLT3 ligand is expressed in almost all leukemic cells, it is assumed that cells are proliferated by a mechanism of autocrine stimulation in leukemia (Meierhoff, G., Leukemia 9, 1368, 1995). Also, FLT3 mRNA has been reported to be expressed in lymphatic leukemic cells and myelocytic leukemic cells (Birg, F., Blood 80, 2584, 1994; Da Silva, N., Leukemia 8, 885, 1994; Brasel, K., Leukemia 9, 1212, 1995; Drexler, H. G., Leukemia 10, 588, 1996). However, there remains unknown how the FLT3 mRNA expression is associated with the pathology of lymphatic leukemia and myelocytic leukemia.
A human FLT3 cDNA has been cloned, and a cDNA nucleotide sequence and the amino acid sequence of the FLT3 protein have been determined [O. Rosnet et al., Blood, 82(4), 1110-1119 (1993)]. The present situation, however, is that the structure and function of FLT during the hematopoietic stem cell differentiation and the malignant alterations to leukemic cells have not been analyzed well.